Question

Is the following pair of statements legal or illegal? Explain why. LABEL: .DATA 3 LABEL: .DATA 4 If it is illegal, will the error be detected during pass 1 or pass 2 of the assembly process?

Short answer

Illegal; error detected in pass 1 due to duplicate labels.

Step-by-step solution

Understand the Syntax

In assembly language, a label is often followed by a directive such as `.DATA`, which is used to define data storage. The label is used as a pointer to the memory location where the data is stored. Each label should uniquely identify a memory location.

Analyze Label Uniqueness

Review the code provided:\[\text{LABEL: .DATA 3} \\text{LABEL: .DATA 4}\]Both statements use the same label, `LABEL`, but point to different memory contents. Labels must be unique within the same scope, so this is a violation of the syntax rules of assembly language.

Determine Legality

Since both statements use the same label, this is illegal because the assembler does not allow duplicate labels. This could cause confusion as to which memory location `LABEL` refers to, making it ambiguous.

Identify Error Detection Pass

The error of non-unique labels is typically detected during pass 1 of the assembly process. During this pass, the assembler builds the symbol table and checks label definitions, ensuring each label is unique before it assigns addresses in pass 2.

Key concepts

Label Syntax

In assembly language, label syntax is crucial for defining pointers to memory locations. Labels act as identifiers, allowing programmers to refer to specific memory addresses for storing or retrieving data. When we look at label syntax, it involves:

• A label name, which is a sequence of characters made up of letters and digits, typically starting with a letter.
• A colon following the label name, signifying its definition.
• A directive or instruction that follows the label.

It's important to remember that labels must be unique within their context. This is because they serve as unique names for memory addresses, ensuring that there is no ambiguity when retrieving or accessing data. In the provided exercise, the label `LABEL` was used twice with different directives, which violates label syntax rules, as each label must be distinct. This ensures that each label can clearly and unambiguously point to a specific data location.

Assembler Process

The assembler process involves converting assembly language into machine code, which the computer's CPU can execute. This process typically occurs in two main passes:

• Pass 1: The assembler reads the assembly code, defines all labels, and builds a symbol table. This first pass checks for syntax errors and ensures all labels and symbols are unique and correctly defined.
• Pass 2: The assembler addresses symbolic instructions and converts them into actual machine code instructions, using the symbol table created during the first pass.

During the first pass, errors such as duplicate labels are identified and flagged. This is crucial because if labels are not unique, it can cause unresolved references or lead to ambiguity, making it impossible for the code to be reliably executed. Hence, most label-related errors, like the one from our original problem, are caught during this initial pass.

Symbol Table

A symbol table is an essential data structure within the assembler process. It acts as a repository that keeps track of labels and symbols, associating them with their memory addresses or data values. Here's how it typically works:

• During the first pass, as the assembler encounters labels, it adds them to the symbol table along with their corresponding addresses.
• Any redefinition of a label results in an error, as each entry in the symbol table should have a unique name, which prevents ambiguities.
• In the second pass, the assembler uses this table to look up addresses for symbolic instructions that need to be converted to machine code.

The symbol table ensures that once data is assigned an address, it can be easily retrieved using its label. In our exercise, the duplicate `LABEL` entries would have caused issues in the symbol table, as maintaining uniqueness is vital for the assembly process to succeed.